The aim of the present double-blind randomized clinical trial is to compare efficacy and adverse effects of neostigmine against fentanyl when used as adjuvant to bupivacaine in caudal anesthesia. A total of 140 children, aged 1-6 year and ASA physical status I, who are scheduled to elective herniorrhaphy, are enrolled. Exclusion criteria are sacral area infection, history of allergic reactions to local anesthetics, bleeding tendency, neurological or spinal disease and lack of parent consent. Patients are assigned, using permuted block randomization method, into four groups of 35. Children in the first group receive a caudal injection of 0.5 ml/kg bupivacaine 0.25% plus fentanyl 1µ/kg. The second group receive 0.5 ml/kg bupivacaine 0.25% plus neostigmine 1µ/kg. Patients in the third group receive 0.5 ml/kg bupivacaine 0.25% plus combination of fentanyl 1µ/kg and neostigmine 1µ/kg, and those in the fourth group only receive 0.5 ml/kg bupivacaine 0.25% concentration. Heart rate (HR), and blood pressure (BP) are recorded every 15 minutes. To assess pain intensity, we use the standard pain questionnaire (Wong-Baker Scale). Hemodynamic changes and drug side effects (nausea and vomiting) are recorded in recovery room and surgery ward in different time points. Also, the time to first analgesic request and the dosage of analgesic agent is recorded. Patients and the researcher who assessed and recorded the data were blinded to group allocation.